Microwave combiner/divider devices are well known to those skilled in the art. These devices consist of a common port and the plurality of input/output ports. If operated as a divider, an input signal is fed to the common port and the input power is divided amongst the plurality of input/output ports which serve as outputs. In the combiner mode, a plurality of inputs are fed to the input/output ports and a single output which combines the power of the various input is present on the common port.
An example of one such device is described in U.S. Pat. No. 2,963,664, issued to Yeagley. This patent discloses a high frequency power dividing apparatus for continuous stepless division of radio frequency power without appreciable phase shift. The device comprises a pair of branch circuits connected in parallel at their input ends to the source, which may be a radio frequency power source. Each of the branch circuits include a first quarter wavelength transformer which may be a linear quarter wavelength coaxial or other type of transmission line, connected directly in series with a second quarter wavelength transformer, such as a second quarter wavelength transmission line having a variable characteristic impedance. The inner conductors of the first transmission line are connected together to the source at one end and at the other end each is connected to one conductor of the second line. The outer conductors of the first lines and the outer conductors of the second lines are connected together, and connected to the other terminal of the source which may be at ground potential. The output of each of the second lines is connected to a load which is preferably purely resistant and has a resistance equal to the impedance of the source, to the characteristic impedance of the first transmission line and also to the maximum characteristic impedance of the second transmission line. By varying the characteristic impedances of these second transmission lines differentially, the power supplied to the load may be given any desired ratio and the impedances of the second lines may be adjusted so that the impedance presented to the source always remains constant.
Similarly, U.S. Pat. No. 4,371,845, issued to Pitzalis, Jr., discloses a modular microwave power divider amplifier combiner. The apparatus includes a power input/output port and a microwave power energy radial divider/combiner section which includes plural microwave energy transmission lines symmetrically radiating from the common port to the outer edge of the divider/combiner section. Each microstrip transmission line includes a microstrip conductor extending from the common input/output port to the outer edge of the divider/combiner section. The apparatus also includes means for transmitting microwave energy between the input/output port and the divider/combiner section.
It should be noted that neither the Yeagley patent nor the Pitzalis, Jr. patent provide for isolation resistors to isolate signals from one input/output port to another. If one of the sources/sinks were to fail, this could greatly disturb the power distribution for each of the functioning input/output ports.
A device which provides for isolation of the input/output ports is described in U.S. Pat. No. 3,091,743, issued to Wilkinson. This patent discloses the power divider that is capable of distributing input radio frequency energy equally between a selected number of output loads. The power divider consists of a coaxial transmission line structure having hollow cylindrical inner and outer conductors, the inner conductor of which is split into a plurality of equal length circumferentially spaced splines. Preferably, the number of splines is equal to the desired number of output channels. All of the splines are shorted together at the input to accommodate a common input terminal, and at the output end, each of the splines is connected to a suitable terminating resistor. The terminating resistors are of identical value, one terminal of each of which is connected to an electrically neutral or floating common junction.
In operation, a signal applied to the input terminal divides equally among the plurality of splines, each of which with the outer conductor, functions as a transmission line. The terminating resistors in conjunction with the splines prevent interaction of the output signals. The input impedance of the divider is matched to the characteristic impedance of each of the output transmission lines when the conditions for isolation are satisfied, and consequently, does not introduce discontinuities in the system in which it is used.
Similarly, U.S. Pat. No. 3,529,265, issued to Podell, discloses a radio frequency power divider, which provides isolation. An input port of characteristic impedance Z.sub.1 is connected across a number of transmission lines (N) arranged such that a serial combination of the input ends of the transmission lines is connected between the two terminals which form the input port. Preferably, N is equal to the power division factor. Thus, if the power divider is to be a three-way divider, then there are three transmission lines. The transmission lines are each one quarter wavelength long at the center frequency of the operating bandwidth and the one conductor of each of the transmission lines is grounded at the end and removed from the input port. The characteristic impedance of each of these transmission lines is selected according to the desired standing wave ratio versus frequency characteristic.
In accordance with the disclosed invention, each one of the transmission lines has connected to it at the end removed from the input port a second quarter wavelength section of transmission line having the same characteristic impedance. One conductor of each of the second group of transmission lines is connected at one end to the non-ground conductor of the corresponding one of the first group of transmission lines. The other end of the conductor of the second transmission line is connected through a terminating impedance to a floating junction. The value of each of the terminating impedances will depend upon the load impedances. The characteristic impedances of the transmission line sections are selected to provide both matching of the second group of transmission lines and isolation of the ports.
U.S. Pat. No. 4,365,215, issued to Landry, also discloses a power divider which provides isolation between input/output ports. The power divider operates over a predetermined frequency band and includes an inner conductor system including a common leg and N-branch legs extending from a common junction with the common leg. The divider also includes an N-terminal resistance element for dissipating odd-mode power and for isolating said N-branch ports from each other with each of the end terminals connected to an associated branch leg at a distance from the common junction of approximately 1/4 wavelength at a frequency in the band. The divider also includes an outer conductor enclosing the inner conductor system and a thermally conducting dielectric heat sink coupled between the resistance element and the outer conductor for conducting heat away from the resistance element. The heat sink between the resistance element and the outer conductor is configured to form a lower dielectric constant than the heat sink between the resistance element and the outer conductor, to provide a lower capacitance between the resistance element and the outer conductor than would be provided in the absence of the lower dielectric constant region. Thus, problems of the prior art, such as limited power handling capabilities, are overcome by placing the isolation resistors inside the outer conductor of the coaxial transmission line on an electrically insulating resistor contact portion of a low capacitance, high thermal conductivity heat sink.
In addition, U.S. Pat. No. 4,721,929, issued to Schnetzer, discloses a multi-stage power divider with isolation. A plurality of passive circuit elements, such as resistors, are arranged to defined a plurality of radial frequency pathways between a power input and a plurality of power outputs, and to divide incoming radio frequency power among the plurality of outputs in a preselected ratio. The passive circuit elements are connected to define a plurality of power-dividing junctions that are located in sequence in at least one radio frequency pathway between the power input and the power output to further divide the radio frequency power and at least one radio frequency pathway. The passive circuit elements also connect the radio frequency power further divided from that one pathway with the radio frequency power in another pathway at a power-combining junction in another pathway, and to provide electrical resistance between the junctions for the further divider power and the adjacent radio frequency pathways. Thus, power division is dependent only upon the line impedances and more practically realizable line impedances are maintained then in conventional broad band dividers, allowing for greater power division ratios than can be implemented in a single resistant power divider.
The systems disclosed in the Wilkinson, Podell, Landry and Schnetzer patents solve the problems of isolation using a series of resistor switches that dissipate any energy created by a failed component. However, each of these systems has a serious resistor cooling problem. Specifically, there is no effective way to remotely locate these load resistors for effective cooling.
U.S. Pat. No. 4,163,955, issued to Iden et al., discloses a cylindrical mode power divider/combiner with isolation which addresses the problem of resistor cooling. The power divider/combiner includes an outer conductor having a longitudinal axis and an impedance transformer means disposed coaxial of the axis and within the outer conductor. Also included is an input/output coaxial transmission line coupled to the transformer means and the outer conductor. First N-discrete, spaced transmission lines supported by a first dielectric cylinder disposed coaxial with the axis and within the outer conductor are also included, each of the first transmission lines being coupled to the transformer means, wherein N is an integer greater than 1. Also included are second N-discrete, spaced transmission lines disposed coaxial of and transverse to the axis remote from the transformer means, each of the second transmission lines being coupled to a different one of the first transmission lines and terminating in a common metallic disk coaxial of and adjacent the axis. The device also includes N-output/input ports each coupled to a different one of the first transmission lines adjacent the transformer means and N-load ports each coupled to a different one of the first transmission lines adjacent the second transmission lines. With the isolating means coupled to ground, this power divider/combiner is advantageous over the Wilkinson device, which includes isolating loads that are "floating." By coupling the loads to ground, the power limitations associated with heat sinking of the resistors in the Wilkinson device are avoided.
U.S. Pat. No. 4,463,326, issued to Hom, also discloses a planar N-way combiner/divider which addresses both isolation and power dissipation. The apparatus, if used as a power combiner, includes a common port at which the combined energy is available. From this common port, a ring-type impedance matching arrangement is included connected to a division point with N microstrip traces radiating therefrom, where N is the number of branch ports provided. From the common point, after impedance transformation, the layout of the microstrip traces is such that alternate branch ports are fed through a path 1/2 wavelength longer than that of the remaining branch port connections. The microstrips circuit traces are so arranged that the desired circuit paths between the division point associated with the common port and each of the branch ports compensates for the phase differentials at the branch ports by virtue of corresponding circuit path length differentials. As disclosed by Hom, isolation resistors are grounded by means of a conventional microstrip quarter wave stub on one end and points along the circuit traces leading from alternate branch ports where the path lengths to the points of resistor connection produce a ORF potential and therefore zero current in the resistors, corresponding to a completely balanced operation. Thus, the prior art limitation respecting power dissipation on the isolation resistors is greatly relieved through the use of the circuit of the present invention.
The patents to Iden and to Hom disclose a power divider/combiner combination which may yield practical cooling of the isolation resistors. However, each of these configurations suffer from a narrow bandwidth which limit the usefulness of the devices in wideband applications.